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Neo4j kernel is a lightweight, embedded Java database designed to store data structured as graphs rather than tables. For more information, see http://neo4j.org.

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/*
 * Copyright (c) 2002-2015 "Neo Technology,"
 * Network Engine for Objects in Lund AB [http://neotechnology.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */
package org.neo4j.kernel.impl.query;

import org.neo4j.helpers.Service;
import org.neo4j.kernel.GraphDatabaseAPI;

public abstract class QueryEngineProvider extends Service
{
    public QueryEngineProvider( String name )
    {
        super( name );
    }

    protected abstract QueryExecutionEngine createEngine( GraphDatabaseAPI graphAPI );

    public static QueryExecutionEngine initialize( GraphDatabaseAPI graphAPI, Iterable providers )
    {
        QueryEngineProvider provider = null;
        for ( QueryEngineProvider candidate : providers )
        {
            if ( provider == null )
            {
                provider = candidate;
            }
            else
            {
                throw new IllegalStateException( "Too many query engines." );
            }
        }
        if ( provider == null )
        {
            return NoQueryEngine.INSTANCE;
        }
        return provider.createEngine( graphAPI );
    }

    public static QueryExecutionEngine noEngine()
    {
        return NoQueryEngine.INSTANCE;
    }

    public static QuerySession embeddedSession()
    {
        final Thread thread = Thread.currentThread();
        return new QuerySession()
        {
            @Override
            public String toString()
            {
                return String.format( "EmbeddedSession{thread=%s}", thread.getName() );
            }
        };
    }
}